Observer pattern in PHP - Mini Project: Build & Apply

Observer pattern

📖 Scenario: You are building a simple notification system where a NewsPublisher sends news updates to multiple Subscribers. When the publisher has new news, all subscribers should be notified automatically.

🎯 Goal: Create a basic Observer pattern in PHP where a NewsPublisher notifies multiple Subscribers when news is published.

📋 What You'll Learn

Create a NewsPublisher class that holds news and a list of subscribers.

Create a Subscriber class that can receive news updates.

Implement methods to add subscribers to the publisher.

Notify all subscribers when new news is published.

Print the news received by each subscriber.

💡 Why This Matters

🌍 Real World

Observer pattern is used in real apps to update parts of a program automatically when something changes, like notifications, event handling, or UI updates.

💼 Career

Understanding Observer pattern helps in building scalable and maintainable software, especially in event-driven programming and frameworks.

1

Create the NewsPublisher class with a subscribers array

Create a class called NewsPublisher with a public property subscribers initialized as an empty array.

PHP

<?php

// Create the NewsPublisher class with a public subscribers array

# Your code here

Hint

Use public array $subscribers = []; inside the class to hold subscribers.

2

Create the Subscriber class with a name property

Create a class called Subscriber with a public property name. Add a constructor that accepts a string $name and sets the name property.

PHP

<?php

class NewsPublisher {
    public array $subscribers = [];
}

// Create the Subscriber class with a name property and constructor

# Your code here

Hint

Use public string $name; and a constructor to set it.

3

Add methods to NewsPublisher to add subscribers and notify them

In the NewsPublisher class, add a public method addSubscriber(Subscriber $subscriber) that adds the subscriber to the subscribers array. Also add a public method notifySubscribers(string $news) that loops over subscribers and calls a receiveNews(string $news) method on each subscriber.

PHP

name = $name;
    }
}

Hint

Use $this->subscribers[] = $subscriber; to add subscribers and a foreach loop to notify.

4

Create instances and test the notification system

Create a NewsPublisher instance called publisher. Create two Subscriber instances with names "Alice" and "Bob". Add both subscribers to publisher using addSubscriber. Call notifySubscribers on publisher with the news string "New PHP version released!". Print the output.

PHP

subscribers[] = $subscriber;
    }

    public function notifySubscribers(string $news): void {
        foreach ($this->subscribers as $subscriber) {
            $subscriber->receiveNews($news);
        }
    }
}

class Subscriber {
    public string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }

    public function receiveNews(string $news): void {
        echo $this->name . " received news: " . $news . "\n";
    }
}

// Create instances and notify subscribers

# Your code here

Hint

Create objects and call methods exactly as described to see the notifications.

Practice

1. What is the main purpose of the Observer pattern in PHP programming?

easy

A. To allow objects to watch and react to changes in another object

B. To speed up the execution of PHP scripts

C. To store data in a database efficiently

D. To handle user input validation

Solution

1. Step 1: Understand the Observer pattern concept

   The Observer pattern is designed to let one object notify others about changes.

2. Step 2: Match the purpose with the options

   To allow objects to watch and react to changes in another object correctly describes this notification and reaction mechanism.

3. Final Answer:

   To allow objects to watch and react to changes in another object -> Option A

4. Quick Check:

   Observer pattern = objects watch and react [OK]

Hint: Observer pattern = watch and react to changes [OK]

Common Mistakes:

• Confusing Observer with database or input handling
• Thinking it speeds up code execution
• Mixing it with unrelated design patterns

2. Which of the following is the correct way to attach an observer to a subject in PHP using the Observer pattern?

easy

A. $subject->addObserver($observer);

B. $observer->attach($subject);

C. $subject->attach($observer);

D. $observer->addSubject($subject);

Solution

1. Step 1: Recall the standard method to add observers

   In PHP's Observer pattern, the subject calls attach() to add observers.

2. Step 2: Verify method and object roles

   The subject manages observers, so $subject->attach($observer); is correct.

3. Final Answer:

   $subject->attach($observer); -> Option C

4. Quick Check:

   Subject attaches observer = $subject->attach($observer) [OK]

Hint: Subject attaches observer with attach() method [OK]

Common Mistakes:

• Calling attach() on observer instead of subject
• Using non-standard method names like addObserver
• Mixing up subject and observer roles

3. Given the following PHP code using the Observer pattern, what will be the output?

$subject = new Subject();
$observer = new ConcreteObserver();
$subject->attach($observer);
$subject->setState('Hello');

Assuming ConcreteObserver prints the new state when notified.

medium

A. Hello

B. State changed

C. No output

D. Error: method not found

Solution

1. Step 1: Understand the flow of notification

   The subject changes state and notifies the observer, which prints the new state.

2. Step 2: Identify the observer's reaction

   The observer prints the state 'Hello' when notified.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Observer prints new state = Hello [OK]

Hint: Observer prints state when notified after attach [OK]

Common Mistakes:

• Assuming no output without calling notify explicitly
• Confusing printed message with generic text
• Expecting errors without method definitions

4. What is wrong with this PHP Observer pattern code snippet?

class Subject {
  private $observers = [];
  public function attach($observer) {
    $this->observers[] = $observer;
  }
  public function notify() {
    foreach ($this->observers as $observer) {
      $observer->update();
    }
  }
}

$subject = new Subject();
$subject->notify();

medium

A. attach() method is missing

B. Observers array is private and inaccessible

C. notify() method should not call update()

D. No observers attached before notify is called

Solution

1. Step 1: Check observer attachment before notify

   The code calls notify() without attaching any observers first.

2. Step 2: Understand effect of no observers

   Since no observers are attached, notify() loops over an empty array and does nothing.

3. Final Answer:

   No observers attached before notify is called -> Option D

4. Quick Check:

   Notify without observers = no action [OK]

Hint: Attach observers before notify() to trigger updates [OK]

Common Mistakes:

• Thinking private array causes error
• Assuming attach() method is missing
• Believing notify() should not call update()

5. You want to implement an Observer pattern in PHP where multiple observers react differently to the same state change. Which approach best supports this requirement?

hard

A. Use a single observer class and change its behavior with if-else inside update()

B. Create different observer classes implementing a common interface and attach them to the subject

C. Attach only one observer and switch its internal state manually

D. Avoid using Observer pattern and use global variables instead

Solution

1. Step 1: Identify how to support multiple reactions

   Different observer classes allow each to react uniquely to the same notification.

2. Step 2: Evaluate options for flexibility and design

   Create different observer classes implementing a common interface and attach them to the subject uses a common interface but different classes, which is clean and scalable.

3. Final Answer:

   Create different observer classes implementing a common interface and attach them to the subject -> Option B

4. Quick Check:

   Multiple observers = different classes with common interface [OK]

Hint: Use multiple observer classes for different reactions [OK]

Common Mistakes:

• Using one observer with complex if-else logic
• Attaching only one observer limits reactions
• Ignoring design principles by using globals